Subject: [mobile-ip] MIN_DELAY_BETWEEN_RAS constant and config variables

From: Jari Arkko <[email-masked]>
Date: Thu, 05 Sep 2002 16:20:22 +0300

Issue 88 (http://www.piuha.net/~jarkko/publications/mipv6/issues/issue88.txt)
was about the need to lower one of the RFC 2461 constants, MIN_DELAY_BETWEEN_RAS
because otherwise we couldn't set the router advertisement interval
(MinRtrAdvInterval) default low enough. So, we agreed that this needs to be
done.

However, Brett has brought up an additional complication. RFC 2461 states:

    "... consecutive Router Advertisements
    sent to the all-nodes multicast address MUST be rate limited to no
    more than one advertisement every MIN_DELAY_BETWEEN_RAS seconds."

This applies to both solicited and unsolicited RAs. In order
to make MinRtrAdvInterval low enough, we must lower MIN_DELAY_BETWEEN_RAS
as low as well. However, as a side effect of this we apparently will
make it possible for also solicited RAs to be generated more often.

Assume that the administrator has not followed the recommended
values (say, due to the link characteristics) and is using a higher
value for MinRtrAdvInterval. Now, since MIN_DELAY_BETWEEN_RAS
is a constant, not a configuration variable, a host that generates
many RSes will be able to generate an RA every 0.05 seconds.

Is this a problem or not? We have the following options:

(1) Just lower the MIN_DELAY_BETWEEN_RAS constant and suffer
     the possibility that hosts induce too many RAs even if the
     config vars are set high.

(2) Make MIN_DELAY_BETWEEN_RAS also configurable (or invent a
     new configuration variable). This ensures that current
     characteristics of RAs can be kept by correct configuration.
     However, a constant needs to change to a configurable variable
     and this looks a bit funny specification-wise ;-)

Opinions?

Jari

---

From: Brett Pentland <[email-masked]>
Date: Fri, 06 Sep 2002 10:58:24 +1000

Jari Arkko wrote:

<snip>

>      and this looks a bit funny specification-wise ;-)

In RFC2461, MIN_DELAY_BETWEEN_RAS is only mentioned in section 6.2.6,
"Processing Router Solicitations".  Although it states "...consectutive
Router Advertisements sent to the all-nodes multicast address MUST be
rate limited to no more than one advertisement every
MIN_DELAY_BETWEEN_RAS seconds.", it does so in the context of replying
to Router Solicitations.

I think that a third option might be to ammend/clarify that the protocol
constant MIN_DELAY_BETWEEN_RAS only applies to _solicited_ multicast
RAs.  In this case it could be left at it's current value of 3 seconds
and MinRtrAdvInterval could be lowered for mobile IPv6 without opening
the router to the aforementioned abuse.

Is that sort of alteration to the operation of ND at all feasible?

Brett.